Flower Structure Guide

Completeness

• Complete flowers: Contain all four floral whorls (sepals, petals, stamens, and carpels)
• Incomplete flowers: Lack one or more of the four whorls

Reproductive Organs

• Perfect (bisexual) flowers: Contain both male and female reproductive organs
• Imperfect (unisexual) flowers: Contain either male or female reproductive organs, but not both

Plant Distribution

• Monoecious plants: Have separate male and female flowers on the same plant (e.g., corn, oak)
• Dioecious plants: Have male and female flowers on separate plants (e.g., holly, willow)

Merosity (Number of Parts)

• Trimerous: Floral parts in multiples of three (common in monocots)
• Tetramerous: Floral parts in multiples of four
• Pentamerous: Floral parts in multiples of five (common in eudicots)

Actinomorphic (Radial Symmetry)

Actinomorphic flowers can be divided into equal halves along multiple planes passing through the central axis, like the spokes of a wheel. These flowers are also called regular or radially symmetrical. Actinomorphic symmetry is considered the ancestral condition in flowering plants and is found in many plant families including Rosaceae (roses), Ranunculaceae (buttercups), Liliaceae (lilies), and Solanaceae (nightshades).

Radially symmetrical flowers typically allow access to pollinators from multiple directions and are often associated with less specialized pollination systems. They may be pollinated by a variety of insects, birds, or even wind. The regular arrangement of floral parts in actinomorphic flowers creates a distinctive star-like or circular appearance when viewed from above.

Zygomorphic (Bilateral Symmetry)

Zygomorphic flowers can be divided into equal halves along only one plane, typically the vertical plane. These flowers are also called irregular or bilaterally symmetrical. Zygomorphic symmetry has evolved independently in many plant lineages and is characteristic of families such as Fabaceae (legumes), Lamiaceae (mints), Orchidaceae (orchids), and Scrophulariaceae (figworts).

Bilateral symmetry is often associated with specialized pollination systems, as it can guide pollinators to approach the flower from a specific direction, ensuring efficient pollen transfer. Zygomorphic flowers typically have distinct upper and lower portions, often with modified petals that form landing platforms, tubes, or other structures that facilitate specific pollinator interactions.

Asymmetric

Asymmetric flowers cannot be divided into equal halves along any plane. These flowers are relatively rare but occur in some plant families such as Cannaceae (cannas) and Marantaceae (prayer plants). Asymmetric flowers often have specialized pollination mechanisms that involve precise positioning of reproductive structures in relation to pollinators.

The asymmetry in these flowers may result from the reduction or modification of certain floral parts, the twisting of the flower during development, or the unequal growth of different regions of the flower. Despite their irregularity, asymmetric flowers often show consistent patterns within a species, making them valuable for identification.

Evolutionary Significance

The evolution of flower symmetry has played a crucial role in the diversification of flowering plants. While actinomorphic symmetry is generally considered the ancestral condition, zygomorphic flowers have evolved independently in multiple lineages, often in conjunction with specialized pollination systems.

Zygomorphic flowers are particularly common in advanced evolutionary lineages and are associated with higher rates of speciation in some groups. The transition from actinomorphic to zygomorphic flowers has occurred repeatedly throughout angiosperm evolution, suggesting that bilateral symmetry provides adaptive advantages in certain ecological contexts, particularly those involving specialized animal pollinators.

Some plant families contain both actinomorphic and zygomorphic members, while others are characterized by a consistent symmetry type. The pattern of symmetry within a family can provide insights into evolutionary relationships and ecological adaptations.

Calyx (Sepals)

The calyx is the outermost whorl of the flower, composed of individual units called sepals. Sepals are typically green, leaf-like structures that protect the developing flower bud, though they may be colored (petaloid) in some species. The number, arrangement, fusion, and persistence of sepals are important identification features.

Key variations in the calyx include:

• Free sepals: Sepals that are separate from each other
• Connate sepals: Sepals that are fused together, forming a tube or cup
• Caducous sepals: Fall off as the flower opens
• Persistent sepals: Remain attached during fruit development
• Accrescent sepals: Enlarge after flowering and may enclose the fruit
• Calyx with specialized structures: Spurs, hooks, bristles, or inflated portions

In some plant families, the calyx is modified into specialized structures such as the pappus in Asteraceae (which aids in seed dispersal) or the inflated calyx in Physalis (Chinese lantern plant) that encloses the fruit.

Corolla (Petals)

The corolla is composed of petals, which are often the most conspicuous parts of the flower. Petals are typically colored and may have patterns, scents, or specialized structures that attract and guide pollinators. The number, arrangement, fusion, and shape of petals are highly diverse and provide valuable identification features.

Key variations in the corolla include:

• Polypetalous (choripetalous): Petals are separate from each other
• Gamopetalous (sympetalous): Petals are fused together, forming a tube, funnel, bell, or other shapes
• Regular corolla: All petals are similar in size and shape
• Irregular corolla: Petals differ in size, shape, or orientation

Specialized corolla types include:

• Papilionaceous: Butterfly-like corolla with standard, wings, and keel petals (Fabaceae subfamily Faboideae)
• Ligulate: Strap-shaped corolla found in some Asteraceae
• Bilabiate: Two-lipped corolla common in Lamiaceae and related families
• Rotate: Wheel-shaped corolla with a short tube and spreading lobes
• Salverform: Corolla with a long, narrow tube and spreading lobes
• Campanulate: Bell-shaped corolla

Androecium (Stamens)

The androecium consists of stamens, the male reproductive organs of the flower. Each stamen typically consists of a filament (stalk) and an anther (pollen-producing structure). The number, arrangement, fusion, and specific morphology of stamens are important for identification and often characteristic of particular plant families.

Key variations in the androecium include:

• Number of stamens: From a single stamen to numerous stamens
• Free stamens: Stamens separate from each other
• Connate stamens: Stamens fused together by their filaments, anthers, or both
• Adnate stamens: Stamens fused to other floral parts, such as the corolla
• Didynamous stamens: Four stamens in two pairs of unequal length
• Tetradynamous stamens: Six stamens, four long and two short (characteristic of Brassicaceae)

Specialized stamen arrangements include:

• Monadelphous: Filaments fused into a single tube (e.g., many Malvaceae)
• Diadelphous: Filaments fused into two groups (e.g., many Fabaceae)
• Polyadelphous: Filaments fused into multiple groups (e.g., Hypericum)
• Syngenesious: Anthers fused while filaments are free (e.g., Asteraceae)

Gynoecium (Carpels)

The gynoecium consists of carpels, the female reproductive organs of the flower. Each carpel typically includes an ovary (containing ovules), a style, and a stigma (which receives pollen). The number, arrangement, fusion, and position of carpels provide crucial identification features and are often characteristic of particular plant families.

Key variations in the gynoecium include:

• Apocarpous: Carpels separate from each other
• Syncarpous: Carpels fused together
• Superior ovary: Ovary positioned above the attachment of other floral parts
• Inferior ovary: Ovary positioned below the attachment of other floral parts
• Half-inferior ovary: Ovary partially embedded in the receptacle

The internal structure of the ovary is also important for identification:

• Locule number: The number of chambers within the ovary
• Placentation: The arrangement of ovules within the ovary (e.g., axile, parietal, free-central, basal)
• Ovule number: From a single ovule to numerous ovules per carpel

The style and stigma also show important variations:

• Style number: From a single style to multiple styles
• Style position: Terminal, lateral, or gynobasic
• Stigma shape: Capitate (head-like), lobed, plumose (feathery), or other forms

Perianth

The perianth refers collectively to the calyx and corolla. In some plants, particularly many monocots, the sepals and petals are similar in appearance and are called tepals. The arrangement and differentiation of the perianth can provide important identification clues.

Key variations in the perianth include:

• Differentiated perianth: Clear distinction between sepals and petals
• Undifferentiated perianth: Sepals and petals similar in appearance (tepals)
• Double perianth: Both calyx and corolla present
• Simple perianth: Only one whorl present (either sepals or petals)
• Apetalous flowers: Petals absent, but sepals present
• Naked flowers: Both sepals and petals absent

Receptacle and Hypanthium

The receptacle is the expanded portion of the flower stalk to which the floral organs are attached. In some plants, the receptacle is modified into specialized structures that contribute to fruit formation or seed dispersal. The hypanthium is a cup-like or tubular structure formed by the fusion of the basal parts of the sepals, petals, and stamens.

Key variations include:

• Flat or convex receptacle: The typical form in many flowers
• Concave receptacle: Forms a cup-like structure
• Elongated receptacle: Creates separation between whorls of floral organs
• Fleshy receptacle: Becomes part of the fruit (e.g., strawberry)
• Hypanthium: Cup or tube formed by fusion of floral parts, characteristic of many Rosaceae and related families

The relationship between the receptacle, hypanthium, and ovary determines whether the ovary is superior, inferior, or half-inferior, which is a key characteristic for family identification.

Indeterminate (Racemose) Inflorescences

In indeterminate inflorescences, the main axis continues to grow and produce new flowers from the base upward, with the youngest flowers at the tip. The main types include:

• Raceme: An unbranched inflorescence with pedicellate (stalked) flowers along the main axis. Examples include foxglove (Digitalis) and snapdragon (Antirrhinum).
• Spike: Similar to a raceme but with sessile (stalkless) flowers attached directly to the main axis. Examples include plantain (Plantago) and wheat (Triticum).
• Spadix: A spike with a fleshy axis, typically enclosed by a specialized bract called a spathe. Characteristic of the Araceae family (arums, peace lilies).
• Catkin: A pendulous spike with reduced, often unisexual flowers. Common in many trees such as willows (Salix), birches (Betula), and oaks (Quercus).
• Corymb: A flat-topped or convex inflorescence where the lower flowers have longer pedicels than the upper ones, bringing all flowers to approximately the same level. Examples include some Brassicaceae and some Hydrangea species.
• Umbel: An inflorescence where all pedicels arise from the same point at the top of the peduncle, like the ribs of an umbrella. Characteristic of the Apiaceae family (carrots, parsley) and some Allium species (onions, garlic).
• Compound umbel: An umbel of umbels, where the primary rays bear secondary umbels (umbellets) rather than individual flowers. Typical of most Apiaceae.
• Panicle: A branched raceme, with flowers on branches of the main axis. Common in many grasses (Poaceae) and lilacs (Syringa).

Determinate (Cymose) Inflorescences

In determinate inflorescences, the main axis terminates in a flower, stopping its growth. Further growth occurs from lateral buds below the terminal flower. The main types include:

• Solitary terminal flower: The simplest form of determinate inflorescence, with a single flower terminating the stem. Examples include tulips (Tulipa) and poppies (Papaver).
• Simple cyme: The main axis ends in a flower, with one or two lateral branches developing below it, each terminating in a flower. Examples include forget-me-nots (Myosotis) and some Geranium species.
• Dichasium (dichasial cyme): A cyme where each axis produces two lateral branches below the terminal flower. Common in many Caryophyllaceae (pink family) such as Stellaria.
• Monochasium (monochasial cyme): A cyme where each axis produces only one lateral branch below the terminal flower. This creates a zigzag pattern as the inflorescence develops.
• Helicoid cyme: A monochasium where the lateral branches all develop on the same side, creating a coiled appearance. Found in some Boraginaceae like Myosotis.
• Scorpioid cyme: A monochasium where the lateral branches alternate sides, creating a zigzag pattern. Common in many Boraginaceae and Hydrophyllaceae.
• Cincinnus: A monochasium that appears to coil in one plane, like a scorpion's tail. Found in some Commelinaceae and Boraginaceae.

Special Inflorescence Types

Some inflorescences have unique structures that don't fit neatly into the indeterminate or determinate categories:

• Capitulum (head): A dense cluster of sessile flowers on a flattened receptacle, often surrounded by involucral bracts, giving the appearance of a single flower. Characteristic of the Asteraceae family (sunflowers, daisies) and some Dipsacaceae.
• Cyathium: A specialized inflorescence found in Euphorbia, consisting of a cup-like involucre containing a single female flower surrounded by several reduced male flowers.
• Hypanthodium (syconium): A hollow, fleshy receptacle with flowers lining the inner surface, with a small opening at the apex. Characteristic of figs (Ficus).
• Verticillaster: A pair of dense cymes in the axils of opposite leaves, giving the appearance of a whorl of flowers around the stem. Characteristic of many Lamiaceae (mint family).

Compound Inflorescences

Many plants have compound inflorescences that combine different inflorescence types at different levels of organization:

• Panicle of cymes: A branched inflorescence where the main arrangement is indeterminate (panicle) but the ultimate units are determinate (cymes). Found in many Hydrangeaceae and some Lamiaceae.
• Thyrse: A compact panicle with a main indeterminate axis but with lateral branches that are determinate cymes. Common in lilacs (Syringa) and horse chestnuts (Aesculus).
• Compound corymb: A corymb where the primary branches themselves bear corymbs rather than single flowers. Found in some Asteraceae and Brassicaceae.
• Compound raceme: A raceme where the primary branches are themselves racemes. Common in many grasses and some Fabaceae.

The complexity of compound inflorescences can make them challenging to categorize, but understanding the basic patterns of flower development and arrangement can help in identification.

Spurs

Spurs are tubular projections from the perianth, usually containing nectar to attract pollinators. They vary in length, shape, and the floral part from which they develop. Spurs may form from sepals, petals, or a combination of floral parts.

Examples of plants with spurred flowers include:

• Aquilegia (columbine): Five spurs formed from petals
• Delphinium (larkspur): A single spur formed from a sepal
• Linaria (toadflax): A single spur formed from the corolla
• Impatiens (touch-me-not): A spur formed from a sepal
• Viola (violet): A spur formed from a petal

Spurs often evolve in response to specific pollinators, with the length and shape of the spur corresponding to the feeding apparatus of the pollinator. Long, narrow spurs may be associated with moths or hummingbirds, while shorter spurs may be accessible to bees or flies.

Corona

A corona is an additional ring of tissue between the corolla and stamens, often forming a crown-like structure. Coronas vary greatly in size, shape, and color, and may function to guide pollinators, protect nectar, or enhance the visual display of the flower.

Examples of plants with coronas include:

• Narcissus (daffodil): A prominent tubular or cup-shaped corona
• Passiflora (passion flower): A complex corona of colorful filaments
• Asclepias (milkweed): A corona of five hood-like structures, each with a horn
• Silene (catchfly): Small scales at the base of each petal forming a simple corona
• Lychnis (campion): Small appendages at the junction of the petal claw and limb

The structure and development of the corona can be important for distinguishing between related genera and species. In some cases, the corona is derived from modified stamens, while in others it represents outgrowths from the petals or other floral parts.

Nectaries

Nectaries are specialized glandular tissues that secrete nectar, a sugar-rich liquid that attracts pollinators. They can occur in various positions within the flower and may be associated with different floral organs. The location and structure of nectaries can be characteristic of particular plant families.

Common types of nectaries include:

• Receptacular nectaries: Located on the receptacle, often as a disc or ring around the base of the ovary (e.g., many Rosaceae)
• Sepal nectaries: Located on or at the base of sepals (e.g., some Ranunculaceae)
• Petal nectaries: Located on petals, often at the base or in specialized structures like spurs (e.g., many Ranunculaceae)
• Staminal nectaries: Associated with stamens or staminodes (e.g., some Liliaceae)
• Gynoecial nectaries: Located on the ovary or associated with carpels (e.g., many Brassicaceae)

In some plants, nectaries occur outside the flower (extrafloral nectaries) on structures such as petioles, stipules, or bracts. These typically function to attract ants or other insects that may protect the plant from herbivores.

Specialized Stamen Structures

Stamens have evolved numerous specialized structures and mechanisms that enhance pollination efficiency or serve other functions. These modifications can be important for identifying particular plant families or genera.

Examples of specialized stamen structures include:

• Poricidal anthers: Anthers that release pollen through terminal pores rather than longitudinal slits, often requiring "buzz pollination" by bees (e.g., Solanum, Cassia)
• Lever mechanism: Stamens that pivot on a connective, depositing pollen on pollinators in a precise location (e.g., Salvia)
• Staminodes: Sterile stamens that may function to attract pollinators, guide them to nectar, or protect other floral parts (e.g., Penstemon, Aquilegia)
• Sensitive stamens: Stamens that move rapidly when touched, enhancing pollen transfer (e.g., Berberis)
• Pollinia: Pollen grains united into waxy masses that are transferred as a unit (e.g., Orchidaceae, Asclepiadaceae)

The diversity of stamen modifications reflects the many ways that plants have evolved to ensure efficient pollen transfer to specific pollinators.

Specialized Carpel Structures

Carpels have also evolved various specialized structures that enhance pollination, protect ovules, or aid in seed dispersal. These modifications can provide important identification features.

Examples of specialized carpel structures include:

• Stylar column: Fusion of styles and stamens into a central column (e.g., Orchidaceae)
• Stylopodium: A disc-like expansion at the base of the styles (e.g., Apiaceae)
• Gynostegium: Complex structure formed by the fusion of stigma, style, and anthers (e.g., Asclepiadaceae)
• Gynobasic style: Style arising from the base rather than the apex of the ovary (e.g., Lamiaceae, Boraginaceae)
• Specialized stigmas: Adaptations for capturing pollen, such as the pollen-trap of Mimulus or the umbrella-like stigma of Sarracenia

Other Specialized Structures

Many other specialized floral structures exist, each with specific functions related to pollination, protection, or other aspects of reproduction:

• Hypanthium: A cup-like or tubular structure formed by the fusion of the basal portions of the sepals, petals, and stamens (e.g., many Rosaceae)
• Epicalyx: A whorl of bracts resembling an outer calyx (e.g., Malvaceae, some Rosaceae)
• Gynophore: A stalk that elevates the gynoecium above the attachment point of other floral parts (e.g., Capparis, Passiflora)
• Androphore: A stalk that elevates the stamens (e.g., some Capparaceae)
• Floral tube: An elongated tubular structure formed by fusion of various floral parts, restricting access to nectar (e.g., many Lamiaceae, Onagraceae)
• Trap structures: Specialized mechanisms that temporarily trap pollinators, ensuring pollen transfer (e.g., Aristolochia, Arum)

These and many other specialized structures reflect the remarkable diversity of floral adaptations that have evolved in response to different pollinators, environmental conditions, and reproductive strategies.

Bee Pollination (Melittophily)

Bee-pollinated flowers typically have the following characteristics:

• Color: Often blue, purple, yellow, or with UV patterns (bee-visible)
• Shape: Often zygomorphic with landing platforms and nectar guides
• Scent: Sweet, fresh fragrances
• Nectar: Hidden in tubes or spurs of moderate length
• Pollen: Moderate to abundant, sometimes with specialized release mechanisms
• Timing: Usually open during the day

Examples of bee-pollinated plants include many Lamiaceae (mints), Fabaceae (legumes), Scrophulariaceae (figworts), and Boraginaceae (borage family).

Butterfly Pollination (Psychophily)

Butterfly-pollinated flowers typically have the following characteristics:

• Color: Bright colors including red, orange, pink, purple
• Shape: Often flat with landing platforms, narrow tubes for proboscis
• Scent: Sweet, often faint
• Nectar: Hidden in long, narrow tubes
• Pollen: Moderate amount
• Timing: Open during the day

Examples of butterfly-pollinated plants include Buddleja (butterfly bush), many Dianthus species (pinks), Phlox, and some Asclepias (milkweeds).

Moth Pollination (Phalaenophily)

Moth-pollinated flowers typically have the following characteristics:

• Color: White or pale colors, visible at night
• Shape: Tubular with long, narrow nectar tubes
• Scent: Strong, sweet fragrance, especially at night
• Nectar: Abundant, deep in tubes
• Pollen: Limited amount
• Timing: Open at dusk or night, sometimes closing during the day

Examples of moth-pollinated plants include Oenothera (evening primrose), Datura (moonflower), Lonicera (honeysuckle), and many orchids with long nectar spurs.

Bird Pollination (Ornithophily)

Bird-pollinated flowers typically have the following characteristics:

• Color: Bright red, orange, or yellow (birds have poor sense of smell but good color vision)
• Shape: Tubular, often pendant, with sturdy structures
• Scent: Little or no fragrance
• Nectar: Abundant, often dilute
• Pollen: Limited amount
• Timing: Open during the day

Examples of bird-pollinated plants include Aquilegia (columbine), Lobelia, Erythrina (coral tree), and many species of Aloe, Agave, and Eucalyptus.

Bat Pollination (Chiropterophily)

Bat-pollinated flowers typically have the following characteristics:

• Color: White, cream, or green, visible at night
• Shape: Large, bowl-shaped, or brush-like, often pendant
• Scent: Strong, musty, fruity, or fermented
• Nectar: Abundant, often dilute
• Pollen: Abundant, often sticky
• Timing: Open at night, often for just one night
• Position: Often borne on trunks or main branches (cauliflory) or suspended away from foliage

Examples of bat-pollinated plants include Agave, Baobab (Adansonia), many Bombacaceae, and some cacti like Carnegiea (saguaro).

Fly Pollination (Myophily)

Fly-pollinated flowers typically have the following characteristics:

• Color: Dull red, brown, or purple, often with mottled patterns
• Shape: Shallow, open, or trap-like
• Scent: Often unpleasant, resembling rotting meat or dung (in carrion flies), or sweet (in hover flies)
• Nectar: Present in some, absent in others
• Pollen: Variable amount
• Timing: Variable

Examples of fly-pollinated plants include Rafflesia, Stapelia, Aristolochia, and some Arum species. Hover flies also pollinate many Apiaceae and Asteraceae with open, accessible flowers.

Beetle Pollination (Cantharophily)

Beetle-pollinated flowers typically have the following characteristics:

• Color: White, cream, or dull green
• Shape: Bowl-shaped, flat, or with chambers
• Scent: Strong, often fruity or spicy
• Nectar: Often absent, replaced by edible tissues
• Pollen: Abundant
• Timing: Variable
• Structure: Often robust to withstand beetle feeding damage

Examples of beetle-pollinated plants include Magnolia, water lilies (Nymphaea), and many primitive angiosperms.

Wind Pollination (Anemophily)

Wind-pollinated flowers typically have the following characteristics:

• Color: Inconspicuous, often green or brown
• Shape: Reduced, often with exposed stamens and feathery stigmas
• Scent: Absent
• Nectar: Absent
• Pollen: Very abundant, light, and dry
• Timing: Often flowering before leaf emergence in woody plants
• Arrangement: Often in catkins, spikes, or panicles that dangle in the wind

Examples of wind-pollinated plants include grasses (Poaceae), sedges (Cyperaceae), many trees such as oaks (Quercus), birches (Betula), and pines (Pinus).